1. Field of the Invention
This invention relates to a method of forming a photocathode and more specifically to a method of forming a variable sensitivity transmission mode negative electron affinity (NEA) photocathode and the resulting structure wherein the sensitivity of the photocathode to white or monochromatic light can be varied by varying the backsurface recombination velocity of the photoemitting material with a modulated electric field.
2. Description of the Prior Art
Efficient electron emission, based upon the concept of NEA, from Cesium or Cesium-Oxygen treated semiconductor surfaces, such as Gallium-Arsenide (GaAs) or other ternary Group III-V element compounds, and Silicon has had a large impact in the area of low light level detection and particularly in scintillation counting, photomultipliers, and imaging devices. These efficient new semiconductor emitters are characterized by their long minority-carrier diffusion lengths and high electron escape probabilities. The emission mechanism involves thermalization of excited electrons, which are produced by photon or other excitation to the conduction band edge with the result that electrons can diffuse distances of several microns before emission. Because of the NEA condition on a heavily p-doped Cesium-Oxygen treated semiconductor surface, within a diffusion length of the surface can efficiently escape into the vacuum.
Photoemitters utilizing NEA has brought to fruitation a new family of photocathodes with greatly improved performance. In particular, photocathodes made from Group III-V compound materials, such as GaAs, GaInAs, and InAsP, have shown substantial advantages over conventional photocathodes in increased yield and longer wavelength response when they are operated in the reflection mode. While the developments in incorporating Group III-V materials as reflection mode photoemitters have been impressive, there still remains the need for high performance transmission mode operation which is highly desirable for many light-sensing device applications. This would have the advantage of providing low cost high performance photocathodes for these devices.
The fabrication of an efficient NEA transmission mode photocathode requires that a thin high quality single crystal p-doped semiconductor photoemitter layer, such as GaAs, be epitaxially grown on a high quality single crystal substrate material which is different from the photoemitter layer, such as GaP or GaAlAs, in order that the substrate material be transparent for the wavelengths of interest. The fundamental absorption edge occurs at photon energies equal to the bandgap of a material. Thus, for transmission mode cathodes, the bandgap determined by material composition for the substrate must be larger than the bandgap of the emitting layer. There are, however, compromises which can be made in the choice of substrates and photoemissive layers which will allow optimization of response over a range of wavelengths of interest. For example, the choice of GaP as a substrate for a GaAs photoemitter provides broad-band response to about 0.93 microns with a short wavelength cut-off around 0.56 microns. The long wavelength response can be extended beyond 0.93 microns by incorporating Indium into the GaAs to form a lower bandgap GaInAs ternary emitting layer.
There are basically three parameters that have a significant bearing on the sensitivity of a transmission mode NEA photocathode such as GaAs on GaP. These parameters are: (1) the diffusion length, (2) the escape probability, and (3) the minority-carrier recombination velocity at the GaAs-GaP interface. The diffusion length is related to the crystalline perfection and purity of the GaAs layer. The escape probability is related to the degree of NEA at the emitting surface that is brought about by the application of the Cesium-Oxygen activating layer. The backsurface recombination velocity is related to the condition at the interface between the GaAs and GaP and is determined to a degree by the amount and direction of band-bending at this interface. For high sensitivity, parameters (1) and (2) must be large in value while parameter (3) must be low.